Road markers with retroreflective material make road markings visible for oncoming vehicles under nighttime conditions as a result of their ability to retroreflect light from the headlights of the vehicles. Road markers are mounted on the surface of a pavement, such as the center line, to define lanes for traffic. Markers are typically mounted on the road in a spaced relationship for guiding traffic. Retroreflective road markers are useful because such markers show up brighter and last longer than conventional glass bead-filled highway paint strips. An example of a retroreflective road marker is disclosed in U.S. patent application Ser. No. 08/092,708, filed Jul. 15, 1993 by Peter A. Spear et al., now U.S. Pat. No. 5,393,728, issued on Feb. 28, 1995.
With perfect retroreflective materials, light rays are reflected essentially towards a light source in a substantially parallel path along an axis of retroreflectivity. However, perfect retroreflectivity is not necessary for many applications. Instead, a balance is necessary between a cone of divergence which provides a degree of divergence which allows enough divergent light to strike the viewer's eye while not having the intensity of the reflected light at the viewer's eye unduly diminished. Where the only source of illumination is the headlights of an automobile on an unlit road, the ability to retroreflect a cone of divergence to the eye of the driver is important for safety reasons.
One type of retroreflective material is an array of cube-corner or prismatic retroreflectors that are described in U.S. Pat. No. 3,712,706, issued to Stamm (Jan. 23, 1973). Generally, the prisms are made by forming a master negative die on a flat surface of a metal plate or other suitable material. To form the cube-corners, three series of parallel, normally equidistance intersecting V-shaped grooves sixty degrees apart are inscribed in the flat plate. The geometry so generated is of a positive configuration. A die of a negative configuration is then produced, usually by electroforming, which is then used to process the desired cube-corner array into a flat plastic surface.
When the groove angle is 70 degrees, 31 minutes, 43.6 seconds, the angle formed by the intersection of two cube faces (the dihedral angle) is 90 degrees and the incident light is reflected back to the source. On a retroreflector used in traffic applications, the dihedral angle is often changed so that the incidental light coming from an automobile headlight is retroreflected into a cone of light which encompasses the driver's eyes.
Further details concerning the structures and operation of cube-corner microprisms can be found in U.S. Pat. No. 3,684,348, issued to Rowland (Aug. 15, 1972), and a method for making retroreflective sheeting is also disclosed in U.S. Pat. No. 3,689,346, issued to Rowland (Sep. 5, 1972), the teachings of which are incorporated by reference herein. The disclosed method is for forming cube-corner microprisms in a cooperatively configured mold. The prisms are bonded to sheeting which is applied thereover to provide a composite structure in which the cube-corner formations project from one surface of the sheeting.
The efficiency of a retroreflective structure is a measure of the amount of incidental light returned within a cone diverging from the axis of retroreflection. Distortion of the prismatic structure adversely effects the efficiency. Furthermore, cube-corner retroreflective elements have low angularity, i.e., the element will only brightly retroreflect light that impinges on it within a narrow angular range centering approximately on its optical axis. Low angularity arises by the inherent nature of these elements, which are trihedral structures having three mutually perpendicular lateral faces. The elements are arranged so that light to be retroreflected impinges into the internal space defined by the faces, and retroreflection of the impinging light occurs by internal reflection of the light from face to face of the element. Impinging light that is inclined substantially away from the optical axis of the element (which is the trisector of the internal space defined by the faces of the element) strikes a face at an angle less than its critical angle, thereby passing through the face rather than being reflected. This is often a problem with road markers because a high profile is necessary for retroreflection of light while a low profile is necessary to avoid damage to automobile tires.
Therefore, a need exists for a retroreflective structure that can effectively retroreflect impinging light while having a low profile.